Show message when rescanning with no library folders or tracks

When users try to rescan the library with no folders configured,
show a helpful message guiding them to add folders instead of
silently doing nothing.

Author: Swarnadip-Kar

src/library/library_decl.h

@@ -21,6 +21,17 @@ enum class FocusWidget {
 };
 
 struct LibraryScanResultSummary {
+    LibraryScanResultSummary()
+            : autoscan(false),
+              numNewTracks(0),
+              numMovedTracks(0),
+              numMissingTracks(0),
+              numNewMissingTracks(0),
+              numRediscoveredTracks(0),
+              tracksTotal(0),
+              noDirectoriesConfigured(false) {
+    }
+
     QString durationString;
     bool autoscan;
     int numNewTracks;
@@ -29,4 +40,5 @@ struct LibraryScanResultSummary {
     int numNewMissingTracks;
     int numRediscoveredTracks;
     int tracksTotal;
+    bool noDirectoriesConfigured;
 };

src/library/scanner/libraryscanner.cpp

@@ -193,15 +193,20 @@ void LibraryScanner::slotStartScan() {
 
     // Recursively scan each directory in the directories table.
     m_libraryRootDirs = m_directoryDao.loadAllDirectories();
-    // If there are no directories then we have nothing to do. Cleanup and
-    // finish the scan immediately.
-    if (m_libraryRootDirs.isEmpty()) {
+    // If there are no directories then we still have to scan independently added tracks.
+    QSet<QString> trackLocations = m_trackDao.getAllTrackLocations();
+
+    if (m_libraryRootDirs.isEmpty() && trackLocations.isEmpty()) {
+        // Nothing to do. noDirectoriesConfigured == true will show the "no dirs" message.
+        LibraryScanResultSummary result;
+        result.autoscan = m_manualScan;
+        result.noDirectoriesConfigured = true;
+
         changeScannerState(IDLE);
+        emit scanSummary(result);
         return;
     }
     changeScannerState(SCANNING);
-
-    QSet<QString> trackLocations = m_trackDao.getAllTrackLocations();
     // Store number of existing tracks so we can calculate the number
     // of missing tracks in slotFinishUnhashedScan().
     m_previouslyMissingTracks = m_trackDao.getAllMissingTrackLocations();

src/mixxxmainwindow.cpp

@@ -1258,6 +1258,15 @@ void MixxxMainWindow::slotLibraryScanSummaryDlg(const LibraryScanResultSummary&
                 QStringLiteral("<br><b>") +
                 tr("%n track(s) in total", nullptr, result.tracksTotal) +
                 QStringLiteral("</b>");
+
+        if (result.noDirectoriesConfigured) {
+            summary = tr("No music directories configured for scanning.") +
+                    QStringLiteral("<br>") +
+                    tr("Add directories in the library preferences.");
+            // reassign summary to avoid showing "Scan took <Empty String>"
+            // which is not relevant in this case - no scan was actually
+            // performed
+        }
     } else {
         if (result.numNewTracks != 0) {
             summary += tr("%n new track(s) found", nullptr, result.numNewTracks) +